This invention relates generally to electrodialysis and, more particularly to electrolyte chemistry control in electrodialysis processing
Electrodialysis is a membrane separation technology in which stacked pairs of selective cationic and anion selective membranes are typically used to segregate increasingly dilute salt streams from concentrated salt streams. Stacks of membrane pairs can be very large and can include 10 to 100 or more pairs of alternating membranes. At one end of the stack, electrochemical reactions are produced by a cathode in electrolyte solution. At the other end of the stack, another reaction is created by an anode in electrolyte solution. In the usual process, the electrolyte stream is separated from the dilute salt and the concentrated salt flows. The electrolyte solution is continuously applied to the electrodes.
Electrodialysis processing is conventionally driven by the hydrolysis of water, which is caused by applying a voltage across an electrode pair. The production of the gases oxygen and hydrogen are well known and thought to follow chemical reactions as in Equations A1 and C1 at the anode (A1) and at the cathode (C1).
                                                                                          H                  2                                ⁢                O                            →                                                                    1                    2                                    ⁢                                      O                    2                                                  +                                  2                  ⁢                                      H                    +                                                  +                                  2                  ⁢                                      e                    -                                                                                                                          E                o                            =                                                -                  1.229                                ⁢                                                                  ⁢                V                                                                        A1        )                                                                                                      2                  ⁢                                      H                    2                                    ⁢                  O                                +                                  2                  ⁢                                      e                    -                                                              →                                                H                  2                                +                                  2                  ⁢                                      OH                    -                                                                                                                          E                o                            =                                                -                  0.8277                                ⁢                                                                  ⁢                V                                                                        C1        )            
FIG. 1 is a schematic of a conventional electrodialysis membrane stack arrangement, generally designated by the reference numeral 10. The electrodialysis membrane stack arrangement 10 includes an anode electrode cell 12, a cathode electrode cell 14 and a membrane stack (also sometimes simply referred to as a “stack”) 16 appropriately disposed between the anode and the cathode cells.
The membrane stack 16 includes alternating cationic selective membranes 20 (and specifically identified by the references 20a, 20b, 20c . . . ) and anionic selective membranes 22 (and specifically identified by the references 22a, 22b, 22c . . . ), beginning and ending with cationic selective membranes 20a and 20f. By the nature of how the selective membranes are alternated, the flow of anions and cations are caused to become concentrated in one cell pair and diluted in an adjacent cell pair. As shown a manifold system is used to isolate the flow of concentrate (in a concentrate manifold 30) from the flow of diluate (in a diluate manifold 32).
The terminal cationic membranes 20a and 20f, located at either end of the cell stack, serve to isolate the cathode within a cathode cell and the anode within an anode cell, each cell being located on opposite sides of the stack (not shown). As shown in FIG. 1, the terminal cationic selective membranes 20a and 20f also isolate a respective flow area where a flow of electrolyte solution is supplied to the electrodes.
As shown in this representation, sodium ions or other cations, can pass through the cation selective membranes 20. However, the cations are rejected by the anion selective membranes 22. Likewise, chloride ions, or other anions can pass through the anion selective membrane 22, but are rejected by the cation selective membranes 20.
FIG. 1 also shows the dynamic balance between all the cells of the electrodialysis stack. Very importantly, sodium (Na+) plays a crucial balancing role in the proper operation of the electrodes. Because each electrode is isolated from in its corresponding electrolyte by a cationic selective membrane, this means that the ionic current is particularly dependent on sodium transport. However, if the cationic membrane allows calcium or magnesium transport, then these ions will pass into the electrolyte solution.
Electrodialysis has been conventionally used to treat light brine (e.g., brine that in general contains less than 1% salt and in some cases salt in a relative amount of as few as few hundred parts per million).
The application of electrodialysis processing to the treatment or processing of highly concentrated brines, especially those that contain a high concentration of soluble calcium, or other multivalent cations, can be particularly challenging.
For example, a practical problem in applying electrodialysis to the treatment of waters with calcium levels in the range of up to or about 100 mg/l is that a significant flux of calcium can occur through the cationic membrane from the stack cell adjacent to the cathode electrolyte cell to cause scale to form on the cathode and cause precipitates of calcium sulfate and other divalent sulfates to form in the electrolyte solution.
Furthermore, if soluble calcium or some other multivalent cation is transported into the electrolyte solution, then this cation can readily increase resistance to ion flow by fouling the electrode cell (specifically at the cathode) by forming precipitated calcium salts such as calcium hydroxide, or calcium sulfate. This greatly reduces the effective amperage and the rate of ion flux in the electrodialysis stack.
The prior art suggests that all cationic membranes within a stack be made of the same material. As such, if cationic membranes allow calcium flow, then soluble calcium will be transported across the cathode isolation membrane, and thus be integrated into the electrolyte solution. If calcium exclusionary membranes are utilized in all cells of the electrodialysis stack, then calcium cannot be collected in the concentrate and it will remain in diluate stream. This would be deleterious to the overall performance of the process.
Moreover, other divalent cations with similar chemistry to calcium, specifically barium, strontium, and radium, may be encountered in certain brines. An example of such a brine is flowback water from natural gas extraction from shale formations. In view thereof, it can be highly advantageous and sought to exclude these compounds, as well as calcium, from the electrolyte.
One widely used electrolyte solution is concentrated disodium sulfate. The aforementioned divalent cations, such as calcium, barium, and radium, are known to have very low solubility in the presence of sulfate. Thus it can be desirable to be able to use the standard disodium sulfate solution as the electrolyte without the danger of precipitating and concentrating unwanted cations in the electrolyte.
In view of the above, there is a need and a demand for improvements in electrodialysis processing. Further, there is need and a demand for improved control of electrolyte chemistry in such processing. Still further, there is a need and a demand for improvements in minimizing cation fouling, particularly, multivalent fouling in electrodialysis processing.